The rotational motion of crossbridges on myosin heads and their interaction with actin play a central role in muscle contraction. Therefore, an understanding of these motions is necessary before the molecular basis of contraction can be understood. Our objectives are to characterize the rotatonal motions of myosin heads in terms of a rotational correlation time and angular restrictions of this motion to similarly characterize the motion of actin filaments reflected in the motion of a spin label rigidly bound to cysteine 373 and to determine which chemical interactions, conformational changes or structural properties control these motions with myosin and actin from vertebrate fast and slow skeletal muscle, cardiac muscle, and smooth muscle. Rotational motion will be studied by conventional and saturation transfer-EPR using spin labels rigidly bound to the proteins being studied.